The switching frequency jitter feature of the TOPSwitch-GX family allows the use of a small, low cost common mode choke for T4 and reduces the value of C8 and C11 needed to meet EN55022 / CISPR22 Class B with good margin.
A safety rated Y capacitor bridges the isolation barrier from the rectified DC rail to output return.
This returns common mode EMI currents generated by the primary and secondary switching-waveforms, reducing conducted EMI.
EMI results are presented in a later section of this document.
Returning the Y capacitor to the DC rail ensures high currents present during line transients are routed away from U1.
The output of T5 is rectified and filtered by D8 and C7.
Inductor L2 and C21 provide additional high frequency filtering.
Resistor R1 and C1 provide snubbing for D8.
Choosing the proper snubber values is important for low zero-load power consumption and for high frequency EMI suppression.
The snubber components were chosen so that the turn-on voltage spike at the D8 anode is slightly under-damped.
Increasing C1 and reducing R1 will improve damping and high frequency EMI, at the cost of higher zero-load power consumption.
Resistors R15 and R16 divide down the supply output voltage and apply it to the reference pin of error amplifier U6.
Shunt regulator U6 drives Optocoupler U7 through resistor R17 to provide feedback information to the U1 CONTROL pin.
The Optocoupler output also provides power to U5 during normal operating conditions.
Components R2, R17, R14, C3 and C16 all play a role in compensating the power supply control loop.
Capacitor C3 rolls off the gain of U1 at relatively low frequency.
Resistor R2 provides a zero to cancel the phase shift of C3.
Resistor R17 sets the gain of the direct signal path from the supply output through U7 and U6.
Components C16 and R14 roll off the gain of the error amplifier (U6).